1. Field of the Invention
The present invention relates to a toner to be used for electrophotography, an image formation method for visualizing an electrostatic charge image, and a toner jet method.
2. Description of the Related Art
As general electrophotography, there has been known a method involving forming a latent image on an image bearing member (photosensitive member), visualizing the latent image by supplying a toner to the latent image, transferring the toner image onto a transfer member such as paper, and fixing the toner image onto the transfer member with heat or pressure to obtain a duplicate.
In order to save power and shorten wait time in an electrophotographic apparatus, an on-demand type fixing device has been put into practical use as a fixing device, the on-demand type fixing device including a combination of a ceramic heater having small heat capacity and a film.
In the above-mentioned fixing device, an attempt has been made to reduce an internal pressure of a fixing nip of the fixing device from the viewpoints of prolonging life and handling various media.
Further, along with the recent increase in printing speed, the time during which a toner and a medium such as paper pass through a nip of the fixing device is becoming short year after year.
Further, in recent years, there are increasing chances that users output graphic images each having a high printing ratio, such as image data and posters captured by digital cameras, mobile terminals, and the like, through use of an image forming apparatus such as a laser printer (LBP).
Against such backgrounds, there has been a demand for a toner which exhibits excellent low-temperature fixability even in the case where an image having a high printing ratio is formed under severer fixing condition in which the time during which a toner and a medium pass through a nip is short and the internal pressure of a fixing nip is reduced.
In order to achieve low-temperature fixing of a toner, there has been reported a toner containing a crystalline resin as well as an amorphous resin as a binder resin.
It has been known that low-temperature fixability can be improved when a crystalline resin is contained as a binder resin because the crystalline resin is melted in the vicinity of a melting point and becomes compatible with an amorphous resin to soften the binder resin.
However, when the compatibility between the amorphous resin and the crystalline resin is too high, the following problem arises. The crystalline resin becomes hard to crystallize in the toner, crystal formation in the toner after production and in a toner image after fixation becomes insufficient, and storage stability thereof reduces.
In contrast, when the compatibility between the amorphous resin and the crystalline resin is low, a crystal of the crystalline resin tends to be formed more easily, but the amorphous resin and the crystalline resin are difficult to be compatible with each other even at a melting point or more. Therefore, it is difficult to improve the low-temperature fixability, in particular, in the case where the time during which a toner and a medium pass through a nip is short and the internal pressure of a fixing nip of the fixing device is low.
In order to solve the above-mentioned problem, there has been known a technology for accelerating crystallization of a crystalline resin to improve heat-resistance storage property of a toner by controlling the cooling rate of a kneaded molten material (Japanese Patent Application Laid-Open No. 2010-122370) or subjecting the cooled kneaded material to heat treatment at particular temperature (Japanese Patent Application Laid-Open No. 2010-152102) during the step of producing a toner.
Certainly, with the above-mentioned technology, toner which contains a crystal of a crystalline resin and which is improved heat-resistance storage property can be obtained.
However, when the toner is remelted during a fixing step, the crystalline resin and the amorphous resin become compatible with each other and do not return to a phase-separated state formed of the crystalline resin and the amorphous resin even when cooled. Therefore, it has been unable to improve the storage stability of transfer paper on which a fixed toner image has been formed. For example, when the transfer paper on which the toner image has been formed is left to stand in an overlapped state or in contact with another member, the toner image adheres to the transfer paper or the member with which the toner image comes into contact in some cases.
Meanwhile, there has been proposed a procedure for accelerating the crystallization of a crystalline resin by adding a crystal nucleating agent to a toner.
As the crystal nucleating agent, there have been proposed an inorganic crystal nucleating agent such as silica (for example, Japanese Patent Application Laid-Open No. 2007-033773) and an organic crystal nucleating agent such as a fatty acid amide (for example, Japanese Patent Application Laid-Open No. 2006-113473).
However, in order to exert the effect of the crystal nucleating agent with an inorganic crystal nucleating agent such as silica, it is necessary to increase the content of the inorganic crystal nucleating agent, with the result that the melt viscosity of the toner increases owing to the filler effect of the inorganic crystal nucleating agent, hence the low-temperature fixability is sometimes impaired.
Further, although the heat-resistance storage property of a produced toner is improved by an organic crystal nucleating agent such as a fatty acid amide, a crystal nucleating agent which is a low-molecular weight compound is segregated on the surface of the toner during the fixing step. As a result, the effect of the crystal nucleating agent becomes insufficient, and the storage stability of a toner image is difficult to be improved.
Further, the crystal nucleating agent contained in a toner is sometimes segregated on the surface thereof with the passage of time, and the chargeability of the toner is degraded in some cases.
Accordingly, there remain a great number of technical problems to be solved for obtaining a toner satisfying both the excellent low-temperature fixability and the excellent storage stability of a fixed toner image, and the conventional toner still has room for improvement.